indicated mph vs actual mpg with fillup

does anyone have any feedback on the indicated mpg on the dash vs the actual mileage miles divided by gallons when doing a fillup? I have seen that the indicated mpg is super generous, from 1mpg to 4 mpg over actual mileage vs gallons. in times we have owned the car (10,000 miles) we have seen the most difference in hwy mpg, showing mid to high 30s with actual 30-33 mpg. I track my mileage on fuely and wonder if persons are entering the dash indicated mpg or the manually figured mpg.